India is planning to deploy an autonomous regional satellite navigation system to cover its territorial footprint and the footprint of its surrounding areas. The purpose of this navigation system is to cater to the needs of both specific users that require a precise service (PS) and also to the needs of civilian users that require a special positioning service (SPS). The overall constellation of the Indian regional navigational satellite system (IRNSS) will have seven satellites, three of which will be in geostationary orbits and four in geosynchronous orbits. The signals will be transmitted in two bands, namely, L5 band (1176.45 megahertz (MHz)) and S1 band (2492.08 MHz). The SPS signal will be modulated by a 1 MHz binary phase shift keying (BPSK) signal, whereas the PS signal will use a binary offset carrier, BOC (5, 2).
Time to first fix (TTFF) is an important parameter for most satellite navigation receivers, and refers to the time taken by a receiver to output a first position solution from power-on. The TTFF parameter has been examined at length and several approaches have been proposed to reduce this parameter. However, most of the approaches have concentrated on augmenting the receiver with data aid to the receiver.
With the drastic improvements in semiconductor technology, the number of physical channels within a receiver is no more a constraint. Several receiver manufacturers have developed receivers with an excess of 200 channels, which exist concurrently. In addition, the receivers support all in view global navigation satellite system (GNSS) satellite signal processing. The modernized signals of a global positioning system (GPS), namely, L2C and L5, and the proposed signals of Galileo and Compass navigation systems have a minimum of at least two frequencies that support civilian applications. In parallel, there exists dedicated access to their military applications. With an assumption of dual frequency, there is a need for reducing the TTFF for civilian applications, and more importantly, for the precision service (PS) users.
To process GNSS signal leading to the navigation solution, top level functionalities of the receiver can be grouped into the following major categories: code and carrier acquisition, signal tracking, data demodulation, measurement generation, and user solution computation. Till recently in GPS, the SPS code was available only on the L1 frequency, that is, at 1575.42 MHz. However, since inception, PS users had codes on both the frequencies. As a consequence of this, PS users had distinct advantages over SPS users. First, the measurements performed on both the frequencies enabled ionospheric delay estimation. Second, if jamming is present on one frequency, the PS users can coast seamlessly on the other frequency. With a growing demand from a civilian segment for code on the second frequency, GPS and emerging GNSS systems have civilian ranging code on dual frequencies by default or triple frequencies (L5-GPS) in certain cases. In addition, current receivers do not have a limitation on the number of channels and thus, dual frequency processing has become a defacto standard.
In a standard dual frequency SPS receiver, when a lock is established on one frequency, by collaborative tracking methods, a direct lock can be established on the second frequency. Following this, data bit synchronization and measurements can be generated on the second frequency. Assuming that the data is the same on both the frequencies, data extraction or processing is typically not performed on the second frequency. For PS receivers, typically lock is first established on SPS code. Subsequently, based on a signature pattern hand over word (HOW), synchronization of the long PS ranging code is achieved. Effectively for a PS user, there will be three channels processing signals from each satellite, namely, two channels for the dual frequency PS measurements and another channel for SPS to provide access to the HOW word. However, the data processing is typically restricted to a single channel.
Information on navigation (NAV) data of the PS service is sparsely available. However, referring to the data sheets of the PS receivers from various manufacturers, TTFF remains the same to that achieved by the SPS service. This implies that the navigation data remains the same for both the services. To date, not much work has been carried out to exploit the advantages of code and carrier diversity.
A study was carried out on a signaling scheme of operational navigation systems with respect to multiple frequencies of operation. Of all the parameters used to compute TTFF, collection time of ephemeris data (Teph) is a major contributor as Teph completely depends on the navigation data structure of a particular constellation and does not depend on the receiver. In addition, TTFF varies based on the various receiver start modes. In general, the start modes can be classified into four categories, for example, cold start, warm start, hot start, and snap start. In cold start, the receiver is powered on without any prior information. This predominantly takes more time to compute the navigation solution as the receiver has to search the signals of all the satellites of a GNSS constellation to obtain a signal lock, demodulate the data bits, and collect the entire navigation data. In warm start, the receiver has access to almanac data, approximate user position and time, which provides an estimate of all the visible satellites. The receiver pre-positions only the visible satellites onto the available channels and attempts to acquire the signals. To this extent, warm start differs from cold start, wherein the initial search time to lock on the satellites is reduced. Typically, the TTFF for cold start and warm start are, for example, about 100 seconds and about 48 seconds respectively. The above described start modes are predominantly meant for open sky applications.
The next two categories of receiver start modes are hot start and snap start modes. These are used in automotive grade receivers, wherein the receiver has access to additional parameters. Specifically, in hot start, the receiver has access to the latest navigation data, that is, ephemeris data, either stored in a memory from the last power-on, or from an external real time aid. As such, the receiver only needs to obtain the time accurately from the satellite. In case of GPS, the hand over word (HOW) has Z-count information or a time parameter, which repeats once every 6 seconds. Thus, with sub-frame synchronization, the receiver will be able to collect time and in turn make measurements. Snap start is the best case for TTFF, wherein all the receiver parameters including clock parameters of the receiver are available at power-on. This category of receiver makes a fundamental assumption that the receiver was recently powered on and the clock estimate propagated internally is valid for signal processing purposes. With this, the receiver achieves instantaneous lock and with word synchronization, the receiver computes user position. Typically, the TTFF for hot start and snap start modes are, for example, about 8 seconds to about 14 seconds, and 2 seconds respectively. The hot start and snap start modes are used for indoor and high sensitivity applications and are receiver dependent. Since the TTFF is comparatively large in both cold and warm start modes, there is a need for minimizing the TTFF in open sky signal acquisition modes. The drawback with the existing operational systems is that the Teph determines TTFF in cold and warm start modes, a parameter missing in the other two modes.
Consider GPS multi-frequency bands and their signaling with an emphasis on the Teph parameter. Presently, there are 31 GPS satellites with signals transmitted on L1 and L2 frequencies. The SPS service is available only on L1, while PS service is available on both. Furthermore, seven of the 31 satellites transmit the L2C signal and only one transmits the L5 signal. As a part of GPS modernization, it is proposed to have GPS L1C signals. In all, a GPS SPS receiver will have access to signals, for example, ranging codes on four frequency bands. At the same time, apart from GPS M-signals, a military receiver will have access to the above four frequency bands with data and two encrypted channels, that is, L1 and L2, P (Y) codes.
Considering GPS L1, L2C and L5 bands, a top-level navigation data design implements data signal streaming, for example, of 5 sub-frames in 30 seconds in the same sequence, for example, sub-frame 1 to sub-frame 5. L1 was designed in the mid-seventies, while L2C and L5 are recent developments. As such, the L2C and L5 have advanced features and have taken into consideration the limitations of L1. Moreover, some of the recent developments in signal processing have also been accounted for in the signal design. Given the extensive use of the legacy L1 signal and with millions of units being produced, a change in the signal structure is not feasible. Currently in GPS, the navigation data rate is 50 bits per second (bps). With the existing GPS multi frequency of operation, the worst case Teph takes about 30 seconds in either single or dual frequencies, or even with military receivers. In order to improve Teph, either the data rate has to be increased or the number of data bits of navigation data should be reduced. In a co-pending patent application titled “Navigation Data Structure Generation and Data Transmission for Optimal Time to First Fix”, an attempt has been made to optimize and structure the navigation data into four sub frames, where it has been demonstrated that Teph can be optimized to about 24 seconds. However, there is a need to achieve lower TTFF without increasing the data rate or without drastically increasing the transmitted signal power.
Therefore, there is a long felt but unresolved need for a satellite navigation receiver, method and navigation data signal configurations for enhancing the time to first fix (TTFF) parameter for precise service (PS) users and special positioning service (SPS) users in a satellite navigation system by exploiting the advantages of code diversity and carrier diversity in navigation signals.